In recent years, in terms of fossil fuel depletion and/or the necessity of CO2 gas reduction, studies have been pursued to produce ethanol as a fuel from biomass such as corncob, rice straw, switchgrass, Erianthus, scrap wood and so on, which have been wasted. Since many thousands of years ago, humans have already had techniques to convert starch into ethanol through fermentation by the action of yeast Saccharomyces cerevisiae. Starch is a polysaccharide composed of glucoses linked via α-1,4 linkages and can be easily degraded by the action of hydrolases present in various organisms. Glucose is the most preferred carbon source for yeast Saccharomyces cerevisiae, and two molecules of ethanol are produced through fermentation from one molecule of glucose. In contrast, biomass contains cellulose or hemicellulose as a polysaccharide.
Among them, cellulose is a polysaccharide composed of glucoses linked via β-1,4 linkages and is in a crystalline structure. Cellulose has some problems, e.g., in that pretreatment is required to disrupt its crystalline structure and in that enzymes required for its degradation, such as cellobiohydrolase I, cellobiohydrolase II and endoglucanase, do not have sufficient activity. However, there is no problem in fermenting cellulose by the action of yeast Saccharomyces cerevisiae, because the sugar produced after degradation is glucose.
In contrast, hemicellulose comprises not only glucose, but also pentoses such as xylose and arabinose. However, yeast Saccharomyces cerevisiae is conventionally unable to ferment these pentoses. For this reason, techniques to allow genes for xylose reductase and xylitol dehydrogenase from xylose-fermentable yeast Pichia stipitis to be highly expressed in yeast Saccharomyces cerevisiae have often been used for xylose fermentation (FIG. 1). Xylose reductase in Pichia stipitis is an enzyme that uses NADPH as a major coenzyme, and one molecule of NADP is generated after reaction. On the other hand, xylitol dehydrogenase is an enzyme that uses NAD as a major coenzyme, and one molecule of NADH is generated after reaction. Thus, as shown in FIG. 1, the balance of NADP/NADPH or NAD/NADH remains unchanged during glucose fermentation, whereas this balance is shifted to increase NADP or NADH during xylose fermentation. This would be responsible for the low yield in ethanol fermentation from xylose.
Many attempts have been made to prepare a mutated xylose reductase whose coenzyme specificity is altered to NADH (Non-patent Document 1), to prepare a mutated xylitol dehydrogenase whose coenzyme specificity is altered to NADPH (Non-patent Document 2), to design an experiment where a gene for the glycolytic enzyme glyceraldehyde triphosphate dehydrogenase with coenzyme specificity for NADPH is introduced from another organism species (Non-patent Document 3), and to express transhydrogenases (which transfer hydrogen between NADPH and NAD or between NADP and NADH) derived from bacteria such as E. coli (Non-patent Document 4), but these attempts have not succeeded in providing sufficient effects.